Apparatus for scale free heating of metals



T. E. DAVIES Feb. 23, 1965 APPARATUS FOR SCALE FREE HEATING GF METALSled June 24. 1963 CONTROL SENSITIVE RESPONSIVE T0 ',FUEL AIR RATIOID-DGB'DD'G@ Y 3,170,681 APPARATUS FOR SCALE FREE HEATING r OFMETALSTheodore E. Davies, Grosse Ile,v Mich., assignor to The North AmericainManufacturing Company, Cleveland, Ohio, a corporation ofpOhio v FiledJune 24, 1963, Ser. No. 289,876 12 Claims. (Cl. 263-40) LThe inventionlrelates to heating of materials in a United States Patent i ICC ' 2upper combustion chamber'l- `An exhaust opening 20 through whichcombustion gases areexhausted from the furnace is provided' in therecuperator -chamber-1S of the enclosure 12.V Y i v l The 'lowercombustion chamber 14 is providedy with an opening 28 therein throughwhich a mixture of fuel ,y and air from nozzle is passed Y'forcombustion in the chamber 14. As shown in FIGURE l, the upper wallnon-oxidizing atmosphere and refers more specifically vto an improved.structure for scale vfree heating of metals.

In the past heating of materials inl a non-oxidizing atmospherehas beenaccomplished through the use of munies, independent gas generators andthe like. The

., prior heating processes and structures'have produced relatively slowheating of material and have beenparticularly inefficient with regardt'o fuel consumption. In addition apparatus. provided in the past` formaintaining pressures and'temperatures-in conjunction with the heatingprocess andfstructures have been complicated and often inefficient orinaccurate.

An object 'isto provide improvedstructure for heatingy material in anoni-oxidizing atmosphere.

Another object'lis to provide structure for 'heating'` material, such asferrous metaLin-a non-oxidizing atmosphere comprising an enclosurehavinga porous hearth therein on which the material to beheatedmay lbepositionedand which:4 separatesthe enclosureinto a lower,

primary'combustion chamber andan upper, secondary combustion chamber,means for introducing fuel and air insufficient to produce completeburning ofl the fuel-into the primary combustion chamber and means forsubsequently introducing theremainder of the air necessary to completeburning'of the Vfuel into Vthe secondary combustion chamber. l

Another vobject is to provide, structure for heating material as `setforth above including means for maintaining the pressure inthe enclosuresubstantially equal to the pressure surrounding the enclosure.

Another-object is to provide, structure forheating y material as setforth above and further including means for maintaining,4 thetemperature inthe primary com` bustion chamber `substantially constantby varying-the ratio of the air and fuel fed thereto.` j l e l 'rAnother objectis to lprovide "improved structure Afor heatingmateria'lina non- 4Vigidizing atmosphere which is simple, economical"andeticient.

Other objects and featuresfof the invention will become apparent' as-thedescription'proceeds, yespecially when taken in conjunction withtheaccompanying drawings, illustrating preferred embodiments of theinvention, wherein: i A v FIGURE 1 is a diagrammatic illustration offurnace structure constructed in accordance with the invention in whichthe heating procesesmay be practiced. Y FIGURE 2 is a diagrammaticillustrationof a modication of the furnace structure illustratedinFIGURE l.

through which material, such as ferrous metal 22 to bev heated, may beintroduced for positioning on the hearth 24 between the lower combustionchamber 14 and the l material `or Ymetal `of the primarycombustionchamber 14 is formed by the: porous hearth M whereby the gaseousproducts of the combustion of a fuel air mixture in` the primarycombustion-chamber 14 pass upwardly through openyings 32 in the poroushearth 24 around material22 `and, into, the 'secondarycombustion chamber16.

The upper or secondary combustion chamber 16 includes an opening 34therein through which air is admitted into the secondary combustionchamber through the perforated air tube 36 of sufficient lquantityl tocomplete burning of the unburned 'fuel passed into the secondary`combustion chamber from the primary combustion chamber 14, as will beconsidered,subsequently. The` Y tube 36 maybe constructed ofrefract'oryor ceramic or a combination thereof and may be d' air'or watercooledasnecessary.V A

J The .secondary combustion chamber lfurtherincluldes the exhaust`openin`g38 at the top "fthereof wherebylthe products of the secondcombustion .lare passed into the recuperator chamber 18 of the enclosure12.` Opening 20 -is 'provided in enclosure 12 tofpermit placing material22 in -the structure v10 as previously indicated and is shielded byllame` curtain structureZl. v Alame curtain. provided by structure 214isof course optionaluand could be replacedbya door. -A Passage 54 isalsoprovided in enclosure 12 and functions in conjunction with thepressure control apparatus 48 as will be considered subsequently. l

The recuperator chamber 18 includes openings 40 and 42 therein throughwhich an air conduitv 44 is passedto provide preheating for thejairsupplied to the nozzle 30 and perforated air tube 36;" The combustionproducts from the secondary combustion chamber'V 16 are drawn throughthe recuperator chamber :18 iin accordance withV Y the pressuredifferential between the eductor stack46 connected to the exhaustopening"26 in the recuperator chamber 18 and thesecondary combustionchamber 16.`

,.The-ratio of fuel supplied tol nozzle 3) through conduit 74 and valve72 to the .air provided by blower 45 through valve is determined by thefuel-air ratio control apparatus 47.. Apparatus such as apparatus 47 areknown and will not be considered in detail herein. Us`

. ually the apparatus 47 senses the fuel flow through valve Y 72Vwhichmay if desired be selected manually and regulates the valve "70 toprovide sutiicient air so that a desired fuel-air ratio is maintained.`The quantity of fuel metered through valve 72 may be regulated by thetemperatureprobe inthe primary combustion chamber 16 through a fuelquantity control 77 if desired. The fuel may be a gas or a finelydividedcombustible liquid, such as distillate oil or the like ifdesired.

Thekpressure in the secondary combustion chamber 16 is maintainedsubstantially equal'to the pressure surrounding the enclosure 12 bymeans of pressure control apparatus 48. The pressure control apparatus48 includes a stack 50 connected by means of the conduit 52 intotheenclosure 12, the temperature sensing device 56 located in stack 50,blower 58 connected to discharge into eductor stack 48 for varying thepressure in the eductor stack and apparatus 60 for controlling theblower in `accordance with the temperature sensed by the temperaturesensing device. The blower 58 in an actual installation may of course beeliminated and air for the eductor stack may be obtained from blower 45.The operation of the,

3,170,681 Y Patented Feb. 23, 1965v apparatus 48 will be considered inthe over-all operation of the furnace structure 10. I

The furnace structure 10, illustrated in FIGURE l is completed with theapparatus 62 for `controllin'gthe temperature in the primary combustionchamber 14. The apparatus 62 includes the temperature sensing device 64positioned in the primary combustion chamber 14, ,the damper 66 in airsupply conduit 44 for controlling the division of air between the nozzle3() and perforated air tube 36 and the apparatus 568 for controlling thedamper 66 in accordance with the temperature sensedv by the temperaturesensing device 64.

The operation of the furnace structure to produce scale free heating ofmaterial such as ferrous metal 22, positionedin the furnace enclosure 12will now be considered. p

A fuel air ratio for complete combustion of fuel, such asaratio of tenparts air to one part fuel is selected and the fuel-air ratiocontrol 47is adjusted so that sufficient air is metered into air supply conduit 44froml blower 4S through valve 70 positioned between the blower 45 andconduit 44 to permit complete combustion of fuel metered through valve72 in fuel supply conduit 74 from a fuel source (not shown).

The air in. conduit 44 is divided in accordance with the position ofthedamper 66 in the conduit. Thus part of the air necessary to completelyburn the fuel from fuel supply conduit 74 passes into primary combustionchamber 14 after mixing in the nozzle 30 with the'fuel. The resultantfuel air mixture is ignited in the primary combustion chamber 14. v

The air supplied to the nozzle 30 is maintained between predeterminedlimits,A such las between forty and sixty percent of the air necessaryto completely burn the fuel supplied through fuel supply conduit 74.Thus the combustion gases produced in primary combustion chamber 14provide a non-oxidizing atmosphere about the material 22 which is heatedthereby on passage of the combustion products upwardly through theopenings 32 in the porous hearth 24.

The temperature of the primary combustion chamber 14 may be controlledby varying the percentage of air necessary for complete combustion ofthe fuel from conduit 74 which is passed through the nozzle 30. Thus tocontrol the temperature in the primary combustion chamber 14 thetemperature therein is sensed by the temperature sensing device 64 whichproduces a signal proportional to the sensed temperature. The signalfrom the device 64 is then used to control the damper 66 vto vary theportion of air from conduit 44 fed to the nozzle 30 and to theperforated air tube 36 by convenient apparatus 68. Many differenttemperature sensing Vdevices 64, dampers 66 and apparatus 68 foroperating the damper 66 from a signal produced by a temperature sensingdevice 64 are well known, the details thereof will not therefore beincluded herein. p

Thus in starting up of the furnace 10 when the temperature in theprimary combustion chamber 14 is low a higher volume of air is fed tonozzle 30 and less of the air passing through valve 7) is passed throughtube 36 into chamber 16. As the chamber 14 approaches a set operatingtemperature however the air-fuel ratio in. chamber 14 is decreased bymeans of the damper 66 and apparatus 68 to provide a cooler ame andmaintain a predetermined temperature in chamber 14.

The combustion gases from the primary combustion chamber 14 includingpartly burned fuel pass into the area of the perforate air tube 36 afterheating the material 22 in a non-oxidizing atmosphere where the fuel isagain burned with sufficient air to complete burning of the remainingfuel. The secondary combustion of the fuel in the secondary combustionchamber 16 provides a sub-k fuel in the secondary combustion chamber 16proceed upwardly in the secondary combustion chamber 16 and into therecuperator chamber 18.

The combustion products in both the primary and secondary combustionchambers 14 and 16 proceed upwardly due to the normal tendency of heatedgas to ascend and due to` the pressure in the eductor stack 46.

During the heating of the material l22 the pressure in the secondarycombustion chamber 16 is maintained substantially equal to the pressureoutside of the enclosure 12 by varying the pressure inthe eductor stack46 in accordance with the temperature sensed by 'the temperature sensingdevice 56 in stack 50. Thus, if the pressure in the secondary combustionchamber 16 becomes too low due to a rapid exhaust ofthe combustion gasesfrom the secondary combustion chamber 16 through the recuperay tor 18and out of the eductor stack 46, which is undesirable in that airfwouldbe drawn through opening 2t) in the furnace enclosure 12 to provide anoxidizing atrnosV phere about the material 22 being heated, air willalso be drawn into Vthe secondary combustion chamber 16 through thestack 5t? and conduit 52 which will produce cooling of the temperaturesensing device 56 in stack 50. Cooling of the temperature sensing device56 will then effect stantial amount of radiant heating of the material22 without producing an oxidizing atmosphere since the products of thecomplete combustion of the'remaining reduction in the air passeclfro'mblowerS into eductor stack 46V by means of apparatus 60 which will raisethe pressure in the secondary combustion chamber 16 until it issubstantially equal 'to the pressure exterior of the enclosure 12 tostop air entering into the secondary combustion chamber 16 throughopenings 20 and 54.

Conversely, if the pressure becomes too high'in the secondary combustionchamber 16, the combustion gases within the furnace will `pass out ofthe opening 20 and stack 5t) through conduit 52 and opening 54. This isundesirable in that unburned'fuel is lost and the air surrounding the'furnace is contaminated. The passing of the gases from the enclosure 12through stack 50 will cause heating of temperature sensitive device 56which in turn will cause the control apparatus 60 to lower the pressurein eductor stack 46 by controlling blower 58. The pressure in thesecondary combustion chamber 16 of the furnace enclosure 12 is thusreduced to prevent escape of the gases from the furnace.

Thus it will be seen that in accordance with the invention there isprovided a structure for heating material, such as but not limited toferrous metalsfor forging and forming in a non-oxidizing atmospherewhereby the heated material will be scale free which process andapparatus is particularly simple, economical and eicient. Thus forexample, due to the complete combustion of the fuel in the process andstructure disclosed forty percent fuel efficiency in scale freeheatingof material is possible in contrast to many installationspresently in 'use which produce only a ten percent fuel efficiency.Twenty percent fuel efficiency is considered good eiciency forindustrial furnaces of the type'under consideration.

The particular speed of heating of the structure of the invention isattributed to the provision of the secondary combustion chamber and theconsequent heating of the metal from both the top and the bottom thereofwith the top heating being by radiation whereby no oxidation of thematerial is provided even with complete fuel combustion. The efliciencyof the process and structure of the invention is attributed to the useof a recuperator and the continued use of a controlled correct fuel-airratio.

A modification of the furnace structure 10 is illustrated in FIGURE 2.The furnace structure 76 illustrated in FIGURE 2 is different from thefurnace structure 10 illustrated in FIGURE l in that the apparatus 78for controlling the pressure in the enclosure 84 has been substituted inthe structure 76 for apparatus 48 in structure 10, and the air nozzles81 have been substituted for the perforated air-tube 36.

similar to furnace structure 10.

Apparatus 78 comprises a temperature sensing device 8 0 positioned overthe opening 82 in the furnace enclosure 84 so that furnace gasesescaping through opemng 82 will vary the temperature of the temperaturesensing de-v vice 80, the damper 86 for controlling the exhausting of inallther respects the .furnace structure 176 isexactly degrees can beproduced at the temperature sensing device v 80 so that a multitude ofcommercial temperature sensing devices 80 would provide a signalsufficient to vary the position of the damper and maintain the pressurewithin` the furnace enclosure 84 and exterior thereto substantiallyequal. f

` Air nozzles 81 may be substituted for the tube 36 in any installationwhere'the hearth width isnot so broad that diffusion of the airtherefrom would permit oxidation of material 93.

While one embodiment of the'present invention and a single modificationthereof has been considered in detail, it -willbe understood that otherembodiments of the invention and modificationsthereof, particularly inthe structureby which the invention is carried out, are contemplated.For example, the hearth illustrated in the modifications of theinvention vshown inV FIGURES l and 2 need not be a at porous hearth, asillustrated, but might be a hearth 92 constructed of porous arches 94having heat transfer material, such as small aluminum oxide balls 96positioned thereon for receiving material to be heated, as shownin'FIGURE 3. Alternatively the hearth may be specially constructed topermit end heating of billets of metal too `laige toY be completelyreceived'in the furnace or the hearth may be constructed for pushingbars 98 thereove such as the hearth 100 illustrated inV FIGURE 4 whereinthe pores 102 are in alignment with and terminate in a V-shaped groove104. Other hearth constructions. both moving and static, could be usedanda plurality of different types of conveyors could be used with thehearth for moving the metal intopand out of the furnaces illustrated. fY

In addition, the tube or tubes 36 of apparatus 10 could,

be vertical or could extend into the chamber 16 in' a plurality ofdirections and l.the nozzle 81 in apparatus 76 could be in;any Wallofthe enclosure 84. The critical factor being that air from tube 36 or`nozzle 81 should not provide an oxidizing atmosphere'for the metal 22and 93.

' Further the apparatus of the invention could be usedl forjheattreating with temperatures in the neighborhood of l600 F. rather thanVfor heating metal for operations such as but not limitedtto forming orforging at ,temperatures in the neighborhood'of 2400 F. and the metalheated may be non-ferrous as well as ferrous. y'Also, it

l will be noted that the pressure control apparatus 48 and and an uppersecondary combustion chamber, means for oxygen necessary to completelyburn the fuel into thei primary combustion chamber for ignition in theprimary combustion chamber to h'eat material placed on the poroushearth, means for drawing the `combustion gases of the first burning ofthe fuel through the porous hearth over,

the material to be heated and into the secondary combustionchamber andmeans for introducing sufficient oxygen into the secondary combustionchamber at a location adjacent to butabove the material to be heated tocomplete the combustion of the fuel in the combustion gases from thefirst burning of the fuel in the primary combustion chamber in thesecondary combustion chamber and to heat the material by radiation.

2. Structure for heating material ina non-oxidizing mary combustionchamber for ignition in the Vprimary.

combustion chamber to heat material placed on the porous hearth, meansfor drawing the combustion gases of the first burning of the fuelthrough the porous hearth over the material to be heated and into thesecondary combustion chamber, means for introducing sufficient oxygeninto the secondary combustion chamber at a lo-` cation adjacent to butabove the material to-be heated to complete the combustion of the fuelin the combustionl gases from the first burning of the fuel in theprimary combustion chamber in the secondarycombustion chamber and toheat the material by radiation and means positioned adjacent thesecondary combustion chamber within the furnace enclosure for preheatingthe oxygen mixed with the fuel with the combustion product from Vthe'sec-Y` ondary combustion chamber. t

3. Structure as set forth in claim 2 wherein`the means` for preheatingthe oxygenL mixed with the Vfuel comprises a recuperator chamberpositioned adjacent the secondary combustion chamber within the furnace`enclosure and in communication with the secondary combustion chamberadjacent the top thereof, an oxygen supplyconduit passing through therecuperator chamber and means for exhausting gases from the recuperatorchamber at a point remote fromthe secondary combustion chamber.

4. Structure for heating material in a non-oxidizing atmospherecomprising a furnace enclosure, a porous hearth extending transverselyof the enclosure dividing the enclosure into a lower primary combustionchamber and an upper secondary combustion chamber, means formeteringfuel and between 4forty and sixty percent of the oxygennecessary to completely burn the fuel linto thel primary combustionchamber for ignition in the primary combustion chamber to heat materialplaced on the porous hearth, means for drawing the combustion gases ofthe first burning of the fuel through the porous hearth over thematerial to beV heated and into the secondary combustion chamber, meansfor introducing sufficient oxygen into the secondary combustion chamberat a location adjacent to but above :the material to be heated tocomplete the combustion of the fuel in the combustion gases from thefirst burning of the fuel in the primary combustion chamber in thesecondary combustion chamber and to heat the materialby radiation, meansoperably associated with the secondary combustion chamber formaintaining the pressure within the furnace enclosure substantiallyequal to the pressure exterior of the'furnace enclosure and meanspositioned adjacent the secondary combustion chamber within the furnaceenclosure for-preheating the metered oxygen that-is mixed with the fuelwith the combustion product from the secondary combustion chamber. l i

n 5. Structure for heating material in a non-oxidizing atmospherecomprising a furnace enclosure, a porous hearth extending transverselyof the enclosure dividing the enclosure into a lower primary combustionchamber and an upper secondary combustion chamber, means for meteringfuel and between forty and sixty percent of the oxygen necessary tocompletely burn .the fuel into the primary combustion chamber forignition in the primary combustion chamber to heat material placed onthe porous hearth, means for drawing the combustiongases of the firstburning of the fuel through the porous hearth over the material to beheated and into the secondary combustion chamber, means for introducingsutlicient oxygen into the secondary combustion chamber at a locationadjacent to but above the material to be heated to complete thecombustion of the fuel in the combustion gases from the first burning ofthe fuel in the primary cornbustion chamber in the secondary combustionchamber and to heat the material by radiation, means connected betweenthe metering means and the primary combustion chamber for controllingthe temperature within the primary combustion chamber, and meanspositioned adjacent the secondary combustion chamber within the furnaceenclosure for preheating the oxygen mixed with the fuel with theVcombustion product from the secondary combustion chamber. v

6. Structure for heating material in a non-oxidizing atmospherecomprising a furnace enclosure, a porous hearth extending transverselyof the enclosure dividing the enclosure into a lower primary combustionchamber and an upper secondary combustion chamber, means for meteringfuel and between forty and sixty percent of the oxygen necessary tocompletely burn the fuel into the primary combustion chamber forignition inthe lower primary chamber to hea-t material placed on theporous hearth, means for drawing the combustion gases of the firstburning of the fuel through the porous hearth over the material to beheated and into the secondary combustion chamber, means for introducingsufficient oxygen into the secondary combustion chamber at a locationadjacent to but above the material to be heated to complete thecombustion of the fuel in the combustion gases from the first burning ofthe fuel in the primary combustion chamber in the secondary combustionchamber and to heat the material by radiation, means operably associatedwith the secondary combustion chamber for maintaining the pressurewithin the furnace enclosure substantially equal to thepressure exteriorof the furnace enclosure, means connected between the metering means andthe primary combustion chamber for controlling the temperature withinthe primary combustion chamber and means positionedadjacent thesecondary combustion chamber within the furnace enclosure for preheatingthe oxygen mixed with the fuel with the combustion product from thesecondary combustion chamber.

7. Structure for heatingmaterial in a non-oxidizing atmospherecomprising a furnace enclosure, a porous hearth extending transverselyof the enclosure dividing the enclosure into a lower primary combustionchamber and an upper secondary combustionV chamber, means for meteringfuel and between forty and sixty percent of the oxygen necessary tocompletely burn the fuel into the primary combustion chamber forignition in the primary combustion chamber to heat material placed onthe porous hearth, means for drawing the combustion gases yof the firstburning of the fuel through the porous hearth over the material to beheated and into 4the secondary cornbustion chamber, means forintroducing sufficient oxygen into the secondary combustion chamber at alocation ad- Y jacent to but above the material to be heated toVcomplete the combustion of the fuel in the combustion gases from thefirstburning of the fuel in the primary combustion chamber in thesecondary combustion chamber and to heat the material by radiation andmeans operably associated with the ,secondary combustion chamber forsure maintaining means comprises a stack connected into the furnaceenclosure, a temperature sensitive device positioned Within the stack,eductor means operable to control the exhaust of secondary combustionproducts from the furnace enclosure and means responsive to saidternperature sensitive device for actuating the eductor means.. f

9. Structure as set forth in claim 7 wherein the pres,- sure maintainingmeans comprises a temperature sensitive device positioned adjacent anopening in the furnace enclosure, means for regulating the exhaust ofcombustion products from the secondary combustion chamber and meansresponsive to the temperature sensitive device for controlling theexhaust regulating means in accordance with the temperature sensed bythe temperature sensitive device.

l0. Structure for heating material in a non-oxidizing atmospherecomprising a furnace enclosure, va porous hearth extending transverselyof the enclosure dividing the enclosure into a lower primary combustionchamber and an upper secondary combustion chamber, means for meteringfuel and between forty and sixty percent of the oxygen necessary tocompletely burn the fuel into the primary combustion chamber forignition in the primary combustion chamber to heat material placed onthe porous hearth, means for drawing the combustion gases of the firstburningof the fuel through the porous hearth over the material to beheated and into the secondary combustion chamber, means for introducingsufficient oxygen into the secondary combustion chamber at a locationadjacent to but above the material to be heated to complete thecombustion of the fuel in the combustion gases from the first burning ofthe fuel in the primary combustion chamber in the secondary combustionchamber and to heat the material by radiation and means connectedbetween the metering means and the primary combustion chamber forcontrolling the temperature within the primary combustion chamber.

11. Structure as set forth in claim 10 wherein the means for controllingthe temperature in the primary combustion chamber comprises atemperature sensitive device positioned in the primary combustionchamber, ratio contro1- ling means operable on actuation to vary thepercentage of oxygen mixed with fuel in the primary combustion chamberand means responsive to the temperature sensi` tive device for actuatingthe ratio controlling means 'in accordance with a sensed temperature inthe primary combustion chamber. i i l v12. Structure for heatingmaterial in a non-oxidizing atmosphere comprising a furnace enclosure,aVv porous hearth extending transversely of the enclosure dividing theenclosure into a lower primary combustion chamber and an upper secondarycombustion chamber, means for metering fuel and between forty and sixtypercent of the oxygen necessary to completely burn the fuel into theprimary combustion chamber for ignition in the lower primary chamber toheat material placed on the porous hearth, means for drawing thecombustion gases of the first burning of the fuel through the poroushearth over the material to be heated and into the secondary combustionchamber, means for introducing sufficient oxygen into the secondarycombustion chamber at a location adjacent to but above the material tobe heated to complete the combustion of the fuel in the combustion gasesfrom the first burning of the fuel in the primary combustion chamber inthe secondary combustion chamber and to heat the material by radiation,means operably associated with the secondary combustion chamber formaintaining the pressure within the furnace enclosure substantiallyequal to the pressure exterior of the furnace enclosure and meansconnected between the metering means and the primary combustion chamber-for controlling the temperature within the primary Combustion chamber;Y

References Cited by the Examiner UNITED STATES PATENTS Weber 40-31McHenry 263-17 Rockwell 263-17 Mawka 236-15 Smith 236-15 Ness 263-15 Xlo CHARLES SUKALO, Primary Examiner. v

i@ Ness et al 263-15 X Rusciano. Rusciano 266-5 Rusciano 263-15 XSchmidt et al. 263-15 Great Britain.

JOHN J. CAMBY, Examiner.

1. STRUCTURE FOR HEATING MATERIAL IN A NON-OXIDIZING ATMOSPHERECOMPRISING A FURNACE ENCLOSURE, A POROUS HEARTH EXTENDING TRANSVERSELYOF THE ENCLOSURE DIVIDING THE ENCLOSURE INTO A LOWER PRIMARY COMBUSTIONCHAMBER AND AN UPPER SECONDARY COMBISTION CHAMBER, MEANS FOR METERINGFUEL AND BETWEEN FORTY AND SIXTY PERCENT OF THE OXYGEN NECESSARY TOCOMPLETELY BURN THE FUEL INTO THE PRIMARY COMBUSTION CHAMBER FORIGNITION IN THE PRIMARY COMBUSTION CHAMBER TO HEAT MATERIAL PLACED ONTHE POROUS HEARTH, MEANS FOR DRAWING THE COMBUSTION GASES OF THE FIRSTBURNING OF THE FUEL THROUGH THE POROUS HEARTH OVER THE MATERIAL TO BEHEATED AND INTO THE SECONDARY COMBUSTION CHAMBER AND MEANS FORINTRODUCING SUFFICIENT OXY GEN INTO THE SECONDARY COMBUSTION CHAMBER ATA LOCATION ADJACENT TO BUT ABOVE THE MATERIAL TO BE HEATED TO COMPLETETHE COMBUSTION OF THE FUEL IN THE COMBUSTION GASES FROM THE FIRSTBURNING OF THE FUEL IN THE PRIMARY COMBUSTION CHAMBER IN THE SECONDARYCOMBUSTION CHAMBER AND TO HEAT THE MATERIAL BY RADIATION.